The operation of printing on material in web form is often implemented by using rotary printing machines which in general terms can be divided into two groups. In one group the diameter of the cylinder for transferring the print image on to the web, which is referred to hereinafter as the transfer cylinder, is adapted to the length of the print image in such a way that the external printing periphery of the cylinder corresponds to an image length or to a multiple of such an image length, possibly with the addition of a pattern repeat spacing. Operatively associated with that transfer cylinder is an impression cylinder around which the web of material to which printing is to be applied is passed. The transfer cylinder and the impression cylinder rotate continuously at a constant speed so that printing machines of that kind have a very high through-put capacity. A disadvantage with such a machine however is that, when changing from one print image to another, it is necessary to replace at least the transfer cylinder if the subsequent print image is of a different length because that then requires a transfer cylinder of a different diameter. That entails a correspondingly high level of complication and expenditure, especially as conversion of the machine in that way also requires a certain amount of time. For that reason printing machines of that kind are only suitable for producing large numbers of the same print image. That requirement however does not always arise.
In contrast, in the second group of printing machines for printing on material in web form, the transfer cylinder is such that the printingly operative peripheral surface can be adapted to different print image lengths without involving a high level of complication and expenditure. That can be achieved for example in offset printing by virtue of the fact that the peripheral surface of the transfer cylinder is formed by a rubber blanket which without difficulty can be replaced and/or easily adapted to the respective print image length. The transfer cylinders of that group of machines however are generally of such a design that their printingly operative peripheral surface extends over less than 360° so that particular precautions generally have to be taken to apply the print images to the web of material without any spacing or only with a small pattern repeat spacing. For that purpose after each working cycle the web has to be appropriately positioned for the respectively following working cycle in order in that way to permit printing on the web of material with small spacings between the individual print images.
In this respect reference may be made to EP 0 018 291 disclosing a machine configuration in which the printing mechanism has a drive roller which operates independently of the actual printing mechanism. A similar consideration also applies in regard to the printing machine of EP 0 159 225 B1 in which the transfer cylinder is admittedly of a completely cylindrical configuration but which also requires an additional drive means for positioning of the web of material.